As metal materials which are excellent in corrosion resistance and abrasion resistance, martensitic stainless steel, work hardening type austenitic stainless steel, precipitation hardening type and other stainless steel, and ferrite and martensite dual-phase structure stainless steel are known.
Martensitic stainless steel is made a martensite structure by quenching, so can be used while made a high strength. In most cases, it is quenched, then tempered.
The hardness is adjusted by the contents of C and N, the quenching conditions (solution heat treatment temperature, time, and cooling rate), and the tempering conditions (temperature and time).
At the time of martensite transformation, expansion of volume occurs in crystal units, so the steel sheet becomes greater in surface roughness. Martensitic stainless steel is high in strength and low in toughness, so reduction of the surface roughness by temper rolling is not easy.
Further, in the process of making martensitic stainless steel high in strength, the steel is quenched from the austenite single-phase region to obtain a martensite single-phase structure. Cr, Mo, and other elements which improve the corrosion resistance narrow the austenite single-phase formation temperature region, so the amounts of addition are limited.
As one example, in SUS420J1 steel, the amount of Cr is defined as 12 to 14%. For this reason, SUS420J1 steel generally only has the minimum extent of corrosion resistance as stainless steel.
As higher Cr martensitic stainless steel, there are SUS429J1 and SUS431. These contain 15.00 to 17.00% of Cr. If these are made martensite single-phase structures, the ductility becomes low, while if they are made a ferrite or austenite phase and martensite dual-phase structure, the corrosion resistance is impaired.
As a representative type of work hardening type austenitic stainless steel, SUS301 may be mentioned.
SUS301 has an austenite structure at the time of solution treatment. Due to the temper rolling after that, it gradually transforms to the work-induced martensite. Due to the increase in the rolling reduction, the work hardening of the two phases further progresses to make the strength higher.
The composition of SUS301 is 17% Cr−7% Ni. Expensive Ni in 7% is required, so the material cost becomes higher.
Further, the amount of deformation martensite is affected by the material temperature at the time of cold rolling, so in general reverse type rolling in cold rolling of stainless steel, near the coil top and bottom where the rolling speed changes, a change occurs in the amount of deformation martensite and the change in hardness becomes large.
Further, SUS301 is large in work hardening, so when cold rolling hot rolled sheet to finish it to the desired thickness, the rolling load force is high. Depending on the cold rolling speed, annealing process will become necessary and the productivity will otherwise become inferior.
As precipitation hardening type stainless steel, SUS630 (17Cr−4Ni−4Cu), 631 (17Cr−7Ni−1.2Al), and other martensite type precipitation hardened steel are the mainstream.
Martensite type precipitation hardened steel is obtained by solution heat treatment, then cooling to room temperature in the process of which making a martensite structure, then aging so as to cause the formation of precipitated phases rich in Cu and cause fine dispersed precipitation of the intermetallic worked NiAl compound and harden the steel.
Martensite type precipitation hardened steel also requires large amounts of expensive Ni, Cu, and other alloy elements so is high in material cost and is an expensive material.
Furthermore, in the production of martensite type precipitation hardened steel, if precipitation hardened phases are formed at other than the final aging process, the toughness of the material will drop and the cold rolling load will increase resulting in cold rolling becoming no longer possible. For this reason, for example, in the hot rolling process, low temperature coiling is required after hot rolling. Occurrence of defects due to poor coiling shape also becomes a problem.
What has been developed to solve these problems is the dual-phase structure stainless steel which has a dual-phase structure of ferrite and martensite as disclosed in PLT's 1 to 4.
Dual-phase structure stainless steel is obtained by cold rolling hot rolled steel sheet of a ferrite and carbonitride structure, then applying dual-phase annealing which heats this to the ferrite and austenite dual-phase region and cools it so as to transform the austenite phase to martensite and obtain a ferrite and martensite dual-phase structure at room temperature and, furthermore, applying temper rolling and aging.
Dual-phase structure stainless steel is developed based on compositions similar to SUS431 and SUS429J1. The chemical compositions are suitably adjusted to adjust the amount of martensite in accordance with the required hardness.
Dual-phase structure stainless steel reportedly is high in strength and large in ductility, has small in-plane fluctuations in strength, and is excellent in shape flatness as features.
Further, a representative ferritic stainless steel, that is, SUS430 steel, is also reported to easily become a ferrite and martensite dual-phase structure by heating to the dual-phase region and cooling.
However, dual-phase structure stainless steel has a lower amount of Cr in the martensite phase than the ferrite phase, so a difference arises between the phases in the corrosion resistance and the corrosion resistance which is obtained in the average composition cannot be sufficiently obtained or the aging due to corrosion will differ between the phases and thereby unevenness of gloss or hue will occur and the beautiful appearance will be impaired.